Multichannel Dolby system and the like are very popular in current audio systems. Those systems emphasize that the original multichannel audios are first encoded into two-channel audios for transmitting, and then returned to the original multichannel audios by a specially designed decoder for playing.
However, if a system has audios of only two channels, using the aforementioned multichannel systems for processing will cause misleading operation and distortion.
Therefore, if a system is to convert a two-channel audio system into a multichannel audio system, a special design is required.